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Primary state prefixes. ZIP Codes are numbered with the first digit representing a certain group of U.S. states, the second and third digits together representing a region in that group (or perhaps a large city) and the fourth and fifth digits representing a group of delivery addresses within that region.

So what do the extra numbers mean? These last 4 digits represent specific delivery routes within delivery areas. This extra detail means an even more precise matching to a more granular level. It could represent a handful of houses on one side of a street, or even a single building that receives a high volume of mail.

What is the four-digit extension? It represents a specific delivery route within that overall delivery area. For PO boxes, the four-digit extension usually includes the PO box number. Including the last four digits means mailings will arrive to their intended destination faster, as well as discounts on bulk mailings.

ZIP & ZIP+4 United States ZIP codes are five numerical digits long. ZIP+4 codes, also known as add-on or plus-four codes, include four additional digits that identify a geographic segment within a ZIP code delivery area. The first digit in a ZIP code represents a defined group of U.S. states.

If prompted for your ZIP code, just enter the three digits of your postal code plus two zeros. So for example, if your postal code is A2B 3C4, the 5-digit number you should enter is 23400. Contact your card issuer if you have any questions.

A 5-digit number that identifies a particular postal delivery area in the United States.

Sensitive personally identifiable information can include your full name, Social Security Number, driver's license, financial information, and medical records. Non-sensitive personally identifiable information is easily accessible from public sources and can include your zip code, race, gender, and date of birth.

Unknown, empty, or otherwise invalid integers are represented by 0. Zip codes will require leading zeros to correctly format 5-digit and zip-plus-4 codes. Zip-plus-4 notation using a plus-4 of all zeros (-0000) is not valid - use the 5-digit instead.